Neutron diffraction determination of the partial structure factors of molten CuCl

Structure of molten NaCl and the decay of the pair-correlations

The structure of molten NaCl is investigated by combining neutron and x-ray diffraction with molecular dynamics simulations that employed interaction potentials with either rigid or polarizable ions. Special attention is paid to the asymptotic decay of the pair-correlation functions, which is related to the small-k behavior of the partial structure factors, where k denotes the magnitude of the scattering vector. The rigid-ion approach gives access to an effective restricted primitive model in which the anion and cation have equal but opposite charges and are otherwise identical. For this model, the decay of the pair-correlation functions is in qualitative agreement with simple theory. The polarizable ion approach gives a good account of the diffraction results and yields thermodynamic parameters (density, isothermal compressibility, Debye screening length, and heat capacity) in accord with experiment. The longest decay length for the partial pair-distribution functions is a factor of ≃2.5 times greater than the nearest-neighbor distance. The results are commensurate with the decay lengths found for the effective restricted primitive model, which are much shorter than those found in experiments on concentrated electrolytes or ionic liquids using surface force apparatus.

Molten alkali halides – temperature dependence of structure, dynamics and thermodynamics

Temperature-induced structural, dynamical and thermodynamic changes reveal novel insights into the mechanism and dynamics of ion transport in molten salts.

Medium-range correlation of Ag ions in superionic melts of Ag2Se and AgI by reverse Monte Carlo structural modelling-connectivity and void distribution

High-energy x-ray diffraction measurements on molten Ag(2)Se were performed. Partial structure factors and radial distribution functions were deduced by reverse Monte Carlo (RMC) structural modelling on the basis of our new x-ray and earlier published neutron diffraction data. These partial functions were compared with those of molten AgI. Both AgI and Ag(2)Se have a superionic solid phase prior to melting. New RMC structural modelling for molten AgI was performed to revise our previous model with a bond-angle restriction to reduce the number of unphysical Ag triangles. The refined model of molten AgI revealed that isolated unbranched chains formed by Ag ions are the cause of the medium-range order of Ag. In contrast with molten AgI, molten Ag(2)Se has 'cage-like' structures with approximately seven Ag ions surrounding a Se ion. Connectivity analysis revealed that most of the Ag ions in molten Ag(2)Se are located within 2.9 Å of each other and only small voids are found, which is in contrast to the wide distribution of Ag-void radii in molten AgI. It is conjectured that the collective motion of Ag ions through small voids is required to realize the well-known fast diffusion of Ag ions in molten Ag(2)Se, which is comparable to that in molten AgI.

The structure of molten CuCl, CuI and their mixtures as investigated by using neutron diffraction

The structure of molten CuCl, CuI and their mixtures (CuCl)(x)(CuI)(1-x) with x = 0.294, 0.576, 0.801 was studied by using neutron diffraction. The results are discussed by reference to the information that is available on the structure of CuCl and CuI from experiment, theory and computer simulation. The comparison points to a need for more realistic models for the CuCl-CuI system which should take into account the presence of chemical bonds that have been found in CuI by the application of ab initio molecular dynamics methods.

Comparison of partial structures of melts of superionic AgI and CuI and non-superionic AgCl

Neutron and high-energy x-ray diffraction analyses of molten AgI have been performed and the partial structures are discussed in detail with the aid of the structural modelling procedure of the reverse Monte Carlo (RMC) technique by comparison with those of molten CuI and AgCl. It is well known that AgI and CuI have a superionic solid phase below the melting point, in which the cations favour a tetrahedral configuration, while solid AgCl has a rock-salt structure with an octahedral environment around both Ag and Cl atoms. Even in the molten states, there is a significant difference between superionic and non-superionic melts. The cation is located on the triangular plain formed by three iodine ions in molten AgCl and CuI, while molten AgCl favours a 90° Cl-Ag-Cl bond angle, which is understood to maintain a similar local environment to that in the solid state. The atomic configurations of the RMC model suggest that the cation distributions in superionic melts of CuI and AgI exhibit large fluctuations, while Ag ions in the non-superionic melts of AgCl are distributed much more uniformly.

A polarizable ion model for the structure of molten AgI

The results are reported of the molecular dynamics simulations of the coherent static structure factor of molten AgI at 923 K using a polarizable ion model. This model is based on a rigid ion potential, to which the many body interactions due to the anions induced polarization are added. The calculated structure factor is in better agreement with recent neutron diffraction data than that obtained by using simple rigid ion pair potentials. The Voronoi-Delaunay method has been applied to study the relationship between voids in the spatial distribution of cations and the prepeak of the structure factor.

The structure of molten salts

The three-dimensional structures of thirteen MX and MX 2 molten salts (M, metal; X, halide) have been modelled from neutron diffraction data by using the Reverse Monte Carlo method. Although the structures are highly disordered the dominant structural symmetries of the short-range order can be determined by using bond angle correlations and spherical harmonic invariants. It is found that cations in the alkali chlorides tend to be octahedrally coordinated by anions (and vice versa), although the large number of ‘vacancies’ leads to coordination numbers lower than six. CuCl has a tetrahedral coordination of anions about cations. In MX 2 melts small cations are octahedrally coordinated except in ZnCl 2 where the coordination is tetrahedral. As the cation size increases the coordination tends to cubic. In all cases the local structural symmetries in the melt are similar to those in the corresponding crystals. It is shown that the ‘pre-peak’ which occurs in the partial structure factor A MM ( Q ) for MX 2 melts with small cations, and which has been associated with intermediate range ordering, arises from a local density fluctuation due to cation ‘clustering’. This is the first time that a visual picture of such intermediate-range order has been obtained.